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San Antonio Mayor Julian Castro, left, who will be the convention keynote speaker, and his twin brother, State Representative Joaquin Castro, who is running for U.S. Congress, are interviewed at the Democratic National Convention in Charlotte, N.C., Monday, Sept. 3, 2012. (AP Photo/Charles Dharapak)

San Antonio Mayor Julian Castro, left, who will be the convention keynote speaker, and his twin brother, State Representative Joaquin Castro, who is running for U.S. Congress, are interviewed at the Democratic National Convention in Charlotte, N.C., Monday, Sept. 3, 2012. (AP Photo/Charles Dharapak)